Asteroseismology of low-mass stars: the balance between partial ionisation and Coulomb interactions

TL;DR

This study investigates how partial ionisation and Coulomb interactions in the outer layers of low-mass stars influence their stellar oscillations, revealing Coulomb effects dominate in stars below 0.6 solar masses and significantly affect their acoustic spectra.

Contribution

It provides new insights into the relative importance of Coulomb interactions versus partial ionisation in shaping the oscillation spectra of low-mass stars, especially below 0.6 solar masses.

Findings

Coulomb effects become dominant over partial ionisation in stars with mass less than 0.6 M_sun.

Coulomb interactions cause strong scatter in acoustic modes of low-mass stars.

Coulomb effects induce oscillatory features in the sound-speed profile with diagnostic potential.

Abstract

All cool stars with outer convective zones have the potential to exhibit stochastically excited stellar oscillations. In this work, we explore the outer layers of stars less massive than the Sun. In particular, we have computed a set of stellar models raging from 0.4 to 0.9 $M_\odot$ with the aim at determining the impact on stellar oscillations of two physical processes occurring in the envelopes of these stars. Namely, the partial ionisation of chemical elements and the electrostatic interactions between particles in the outer layers. We find that alongside with partial ionisation, Coulomb effects also impact the acoustic oscillation spectrum. We confirm the well-known result that as the mass of a star decreases, the electrostatic interactions between particles become relevant. We found that their impact on stellar oscillations increases with decreasing mass, and for the stars with the lowest masses ($M \lesssim 0.6 M_\odot$), it is shown that Coulomb effects dominate over partial ionisation processes producing a strong scatter on the acoustic modes. The influence of Coulomb interactions on the sound-speed gradient profile produces a strong oscillatory behaviour with diagnostic potential for the future.